In the optical communication field or the optical measurement field, an optical waveguide element such as an optical intensity modulator having optical waveguides and modulation electrodes formed on a substrate having an electro-optical effect such as lithium niobate (LN) is being widely used.
In a substrate having an electro-optical effect, a so-called DC drift phenomenon in which the accumulation of charges generated by polarization or charges generated inside the substrate in response to an externally-applied electric field changes the driving voltage of an optical waveguide element occurs. In addition, a temperature drift phenomenon caused by the temperature change of the substrate also occurs.
Meanwhile, in forming the optical waveguides on the substrate and also disposing the modulation electrodes on the substrate, it is necessary to suppress the absorption and scattering of optical waves propagating through the optical waveguides by the modulation electrodes. Therefore, a buffer layer such as SiO2 is disposed between the substrate having the optical waveguide formed thereon and the modulation electrodes.
Patent Literature No. 1 discloses the lamination of a first buffer layer, a conductive layer, a second buffer layer, and a center conductor disposed on a substrate in this order to suppress DC drift or temperature drift.
However, in a case in which a conductive film is formed on the surface of the buffer layer as a charge dispersion film, it becomes difficult to effectively disperse charges accumulated between the substrate and the buffer layer. Therefore, the accumulated charges have caused a problem of a change in the operating point of a product.
Patent Literature No. 1 also discloses that the conductive layer present between the first buffer layer and the second buffer layer is in contact with the substrate having an electro-optical effect. However, the conductive layer having the above-described structure has a graded structure, and thus is required to be produced in a so-called interaction portion region in which an electric field formed by the modulation electrodes acts on the optical waveguides.
Therefore, the production of the above-described graded structure to be homogeneous and the control of the respective film thicknesses of the first and second buffer layers and the conductive layer respectively lead to difficulties in the manufacturing process. In addition, the conductive layer having a graded structure which is present in the vicinity of the interaction portion region causes the variation in the characteristics of the obtained optical modulator.